Twin dual mode filters with reflectors between transducers being electrically connected between DMS tracks

ABSTRACT

Twin dual mode filter with DMS filters have an input and output transducer (2; 12) per DMS track (A, B) within reflectors (3, 3) and further reflectors (7, 11) between the transducers that are respectively interconnected to one of the transducers. The reflectors (7, 11) of the input or, respectively, output side are respectively connected to one another via interconnects (8, 8). The DMS tracks (A, B) are frequency-shifted relative to one another such that the high-frequency resonance of the low-frequency track (B) falls onto the low-frequency resonance of the high-frequency track (A).

BACKGROUND OF THE INVENTION

Longitudinal mode resonator filters, also called dual mode filters orDMS filters, are referred to below as surface wave filters referencedOFW filters that comprise two normal finger transducers arranged withinreflectors according to the schematic plan view according to FIG. 1 andthat are effective as input or, respectively, output transducer 2, 12.They have two-pole filter characteristics. When two DMS tracks A and Bthat are frequency-shifted relative to one another such that thehigh-frequency resonance of the low-frequency track B falls onto thelow-frequency resonance of the high-frequency track A are interconnectedas shown in FIG. 1, then what is referred to as a twin dual mode filteris obtained, this being distinguished by three-pole filtercharacteristics and by an enhanced edge steepness compared to the singletrack.

Given the filter structure according to FIG. 1, a respective bond wireat the side of the filter input and filter output IN or, respectively,OUT must be conducted at least across one connecting line, across one ofthe DMS tracks A, B in the worst case. This requires additional chip or,respectively, substrate area for the bond pads 5 connected to thetransducers 2, 12 via interconnects 4 between the DMS tracks A, B andcomplicates the fabrication of such a filter.

It is also disadvantageous that the impedances of the filter can be madearbitrarily smaller by enlarging the aperture since disturbingtransverse modes occur in this case.

Given symmetrical operation of the twin dual mode filter, over and abovethis, no optimum suppression of the electrical crosstalk is achievedsince the geometry is not symmetrical.

When the DMS tracks are not optimally aligned relative to one anotherwith respect to the resonant frequencies, then increased amplitude andgroup running time distortions occur in the filter transmission band.Such a de-adjustment is unavoidable in mass production due to thescatters of the manufacturing parameters and the inhomogeneities on thewafer.

SUMMARY OF THE INVENTION

The present invention is based on the object of specifying a twin dualmode filter according to the species initially sited wherein at leastthe first-cited problem is solved, i.e. a crossing-free layout iscreated for the bond wires.

In general terms the present invention is a twin dual mode filter,having two interconnected DMS filters with two normal finger transducersas input or, respectively, output transducers per DMS track withinreflectors. The DMS tracks are frequency-shifted relative to one anothersuch that the high-frequency resonance of the low-frequency track fallsonto the low-frequency resonance of the high-frequency track. Tworeflectors respectively interconnected with one of the transducers arearranged between input and output transducers of each DMS track. Thereflectors of the input or, respectively, output side are respectivelyelectrically connected to one another via interconnects.

Advantageous developments of the present invention are as follows.

The reflectors are replaced by fingers having a suitable width.

Two twin dual mode filters are connected in parallel.

Two twin dual mode filters have series-connected outputs.

Two twin dual mode filters have inputs connected in series and outputsconnected parallel.

Ohmic resistors are connected into the leads to the input and outputtransducers.

Two reflectors respectively interconnected with one of the transducersand having fingers inclined relative to the propagation direction of thesurface waves are arranged between input and output transducers of thelow-frequency DMS track.

Terminal elements of the filter input and filter output are appliedintersection-free onto the substrate surfaces of the DMS tracks facingaway from one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 depicts a prior art longitudinal mode resonator filter;

FIG. 2 an embodiment of an inventive twin dual mode filter in aschematic plan view;

FIGS. 3-6 further embodiments of inventive twin dual mode filters in theillustration according to FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a twin dual mode filter that has two DMS filtersinterconnected via interconnects 4 that comprise two normal fingertransducers within reflectors 3, 3 effective as input or, respectively,output transducer 2 or, respectively, 12 per DMS track A, B. The DMStracks A and B are frequency-shifted relative to one another such thatthe high-frequency resonance of the low-frequency track B calls onto thelow-frequency resonance of the high-frequency track A. Suitable shortreflectors 7, 11 that are respectively interconnected with one of thetransducers 2 or, respectively, 12 are respectively arranged betweeninput and output transducer 2 or, respectively, 12 of each DMS track Aor, respectively, B. The reflectors of the input and output sides arerespectively electrically connected to one another via interconnects 8,8.

As a result of these reflectors and their interconnection with oneanother, the bond pads 5 can be relocated to the outside of the filterstructure and, thus, a layout free of crossings can be obtained.

Both in this as well as in all other embodiments of the invention, thereflectors 7, 11 can be replaced by fingers having a suitable width.

Low impedances can be achieved by circuiting a plurality of DMS trackpairs A, B parallel without the occurrence of disturbing transferse[sic] modes. For example, by connecting two twin dual mode filters inparallel according to FIG. 3, a halving of the impedance is thusobtained.

According to FIG. 4, for example, a balancing of the filter geometry asrequired for optimum suppression of the electrical crosstalk givensymmetrical operation is achieved by a series circuit of two twin dualmode filters. The input-side reflectors 3 of the DMS tracks A and theoutput-side reflectors 3 of the DMS tracks B are thereby separatedaccording to FIG. 3 into voltaically decoupled reflectors 15, 16 or,respectively, 17, 18.

As a result of the series circuit, a doubling of the input impedance isalso achieved given simultaneous halving of the static filtercapacitance. This filter, moreover, can also be asymmetrically operated.

As already mentioned, increased group running time distortions in thefilter transmission band caused by de-adjustment of the DMS tracks A, Bare produced by interaction of the nearly identical resonances of thetwo DMS tracks A, B. These resonance effects can be diminished when thequalities of the resonances are reduced. Accepting an increase of theinsertion attenuation, the qualities can be deteriorated according toFIG. 5, for example, in that ohmic resistors 10 are inserted into theleads to the input and output transducers 2 or, respectively, 12.

Conditioned by the design, the high-frequency resonance has a clearlyhigher quality than the low-frequency resonance given twin dual modefilters. Given maladjustments in the twin dual mode filter, the grouprunning time distortions can also be reduced in that the quality of thehigh-frequency resonance of the low-frequency DMS track B isintentionally reduced. As shown in FIG. 6, this can be inventivelyachieved in that two reflectors 20, 21 respectively connected to one ofthese transducers and having fingers inclined or, respectively, placedat a slant relative to the propagation direction of the surface wavesare arranged between input and output transducer 2 or, respectively, 12of the low-frequency DMS track B. As a result of their slantingattitude, a small part of the acoustic power passing through thesereflectors is reflected out of the acoustic track, for example into awave sink.

The low-frequency resonance arises in the input transducer 2. Theacoustic wave passes through the obliquely placed reflectors only once,whereby it is only weakly damped. Given the high-frequency resonance, bycontrast, the acoustic wave is reflected back and forth between inputand output transducer 2 or, respectively, 12 many times, i.e. that ispasses through the reflectors 20, 21 very often and is thereby highlydamped.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A twin dual mode filter having an input side andan output side, comprising:two interconnected DMS filters having DMStracks and having two normal finger transducers as input or,respectively, output transducers per DMS track within reflectors, theDMS tracks being frequency-shifted relative to one another such that ahigh-frequency resonance of a low-frequency track falls onto alow-frequency resonance of a high-frequency track; two reflectorsrespectively interconnected with one of the transducers and arrangedbetween the input and output transducers of each DMS track thereflectors of the input side or, respectively, output side beingrespectively electrically connected to one another via interconnects. 2.The twin dual mode filter according to claim 1, wherein the reflectorsare fingers having a predetermined width.
 3. The twin dual mode filteraccording to claim 1, wherein two twin dual mode filters are connectedin parallel.
 4. The twin dual mode filter according to claim 1, whereintwo twin dual mode filters have series-connected outputs.
 5. The twindual mode filter according to claim 1, wherein two twin dual modefilters have inputs connected in series and outputs connected parallel.6. The twin dual mode filter according to claim 1, wherein ohmicresistors are respectively connected into leads to the input and outputtransducers.
 7. The twin dual mode filter according to claim 1, whereintwo reflectors respectively interconnected with one of the input andoutput transducers and arranged between input transducer and outputtransducer of the low-frequency track have fingers inclined relative toa propagation direction of surface waves in the filters.
 8. The twindual mode filter according to claim 1, wherein terminal elements of afilter input and a filter output of the filter are appliedintersection-free onto substrate surfaces of the DMS tracks facing awayfrom one another.
 9. A twin dual mode filter system comprising:first andsecond twin dual mode filters, each of said filters havingtwointerconnected DMS filters having DMS tracks and having two normalfinger transducers as input or, respectively, output transducers per DMStrack, the DMS tracks being frequency-shifted relative to one anothersuch that a high-frequency resonance of a low-frequency track falls ontoa low-frequency resonance of a high-frequency track; two reflectorsrespectively interconnected with one of the transducers and arrangedbetween the input and output transducers of each DMS track, thereflectors of the input side, or, respectively, output side beingrespectively electrically connected to one another via interconnects.10. The twin dual mode filter system according to claim 9, wherein thereflectors are fingers having a suitable width.
 11. The twin dual modefilter system according to claim 9, wherein the two twin dual modefilters are connected in parallel.
 12. The twin dual mode filter systemaccording to claim 9, wherein the two twin dual mode filters haveseries-connected outputs.
 13. The twin dual mode filter system accordingto claim 9, wherein the two twin dual mode filters have inputs connectedin series and outputs connected in parallel.
 14. The twin dual modefilter system according to claim 9, wherein ohmic resistors areconnected into leads to the input and output transducers.
 15. The twindual mode filter according to claim 9, wherein two reflectorsrespectively interconnected with one of the transducers and arrangedbetween input and output transducers of the low-frequency DMS track havefingers inclined relative to the propagation direction of the surfacewaves.
 16. The twin dual mode filter system according to claim 9,wherein terminal elements of filter input and filter output of arespective twin dual mode filter are applied intersection-free ontosubstrate surfaces of the DMS tracks facing away from one another.